Carburettors for two-stroke or four-stroke engines are arranged in a suction channel leading to the engine body proper. Often, the carburettor has two air-regulating valves, viz. one throttle valve or air throttle and one choke valve. The throttle valve is used to control the throttle whereas the choke valve is used for cold starts. Usually, the choke valve closes entirely manually or automatically. A small opening in the choke valve ensures that only a minute amount of air flows past the valve upon starting attempts. As a result a very powerful negative pressure generates inside the suction channel adjacent the downstream nozzles. Consequently, a lot of fuel is supplied, giving a very rich air fuel mixture. This is necessary, since large quantities of fuel condensate on the walls of the suction channel, the crank case and the combustion chamber under these conditions. Carburettors do also exist that are equipped with one single air-regulating valve which in this case acts both as a throttle valve and a choke valve. Further, there are injection systems having a separate starting system including choke valves and starter nozzles that work in accordance with the carburettor principle. During start-ups the fuel amount is heavily influenced by the negative pressure from the choke valve.
It is characterstic of these kind of fuel supply system that during cold starts an air-regulating valve, usually designated choke valve, is almost fully closed whereby a heavy negative pressure is formed downstream of the air-regulating valve. Very often the valve is fully closed and a small opening ensures the required by-pass of a small amount of air. Obviously, it is also possible to use an end-position stop means for the air-regulating, valve whereby the latter will gape slightly open and no air opening is required. The size of the minute air opening is dimensioned to suit the lowest starting temperature at which the product usually is used. In the case of chainsaws this temperature could be e.g. -25.degree. C. This means that at higher temperature levels, for instance +20.degree. C., the air opening is too small to provide an accurate fuel-air mixture at start-ups. This gives an unnecessarily rich fuel mixture and complicates or prevents the starting function. In addition, the rich fuel mixture leads to unnecessary engine sooting and unnecessarily large exhaust emissions. For when the product is started the choke is turned to full open, since in reality it is hardly possible to "finely tune" the controls ensuring that the air opening coincides with the demand.